In recent year, Inter-Net has prevailed dramatically in the world, which promotes to prepare information-related infrastructure, through which everybody can get necessary information at any place and anytime. As an interface between this infrastructure and users, a mobile information tool (hereinafter referred to as "MIT") plays a key role.
The MIT, easy to carry, is desirably thin in size, light in weight, and requires a display driven by low power. A reflection type liquid crystal display device (hereinafter referred to as "reflection type LCD"), which is free from a back light, is mostly suitable for this application.
Actually, more than half of the commercialized MITs employ the reflection type LCD. When environments, such as low-power-consumption CPUs, high-speed-communications, OS for portable terminals are well prepared, the MIT business looks promising. The reflection type LCD thus will play more important role as a major component of the MIT. In most of conventional reflection type LCDs, polarizing plates are disposed outside an upper and lower glass substrates, and reflectors are placed on top of that. However, another reflection type LCD, where the polarizing plates are eliminated and a mirror-reflector-electrode is disposed in a cell of LCD and light scatter LCD is used as a modulating layer, is proposed in the following document. This proposed type LCD is expected to improve a reflection factor and accommodate color-LCDs. (written by T. Sonehara, M. Yazaki, H. Iisaka, Y. Tsuchita, H. Sakata, J. Amako, and T. Takeuchi in SID 97 DIGEST, page 1023-1026 published in 1997)
Since light travels free from being absorped because of no polarizing plates, this newly proposed LCD can display a brighter image, and also when a color filter is used to accommodate the color LCD, this LCD is free from color-purity-degradation due to color mixture. The reason is because the reflector electrode is disposed in the cell so that parallax due to a thickness of the glass can be eliminated.
The conventional reflection type LCD having no polarizing plates is described hereinafter with reference to the accompanying drawings.
FIG. 8 is a cross section showing a structure of a reflection type monochrome LCD where a thin-film-transistor (TFT) drives a light scattering LCD. Gate electrode 210 is selectively formed on substrate 201, and gate-insulating layer 211 is formed on top of that to cover gate electrode 210. Semiconductor layer 212 is island-likely formed just above gate electrode 210 and on gate-insulating film 211. Then, source electrode 213 and drain electrode 214 are formed, whereby a TFT, i.e. a switching element, is constructed.
Reflector electrode 202, i.e. a pixel electrode, has a mirror-finished surface, and is coupled electrically to drain electrode 214 via inter-layer insulating layer 215.
Light scattering LCD 203 is sandwiched by substrate 210 and opposite transparence substrate 205 in which a transparent electrode is formed.
Light scattering LCD 203 is formed by curing a mixed system of liquid crystal material having refractive-index-anisotropy and acrylic polymer material. The thickness of the mixed-system-liquid-crystal is optimized, and mixing ratio of liquid crystal vs. polymer material is also optimized so-that the light scattering can be adjusted basically to be forward scatter.
For instance, when refractive index of liquid crystal material vs. regular light is "n.perp.", and that vs. abnormal light is "n.parallel.", and the relation of .DELTA.n=n.parallel.-n.perp.&gt;0 is satisfied, the refractive index of the polymer material is set at approximately same as "n.perp.". In this case, light scattering liquid crystal takes a form of scattering when power is OFF, and a form of transparence when the power is ON.
The reflection type LCD having the construction discussed above is called "polarizer-free type", through which light passes no polarizer, while in the conventional two-polarizer-type, light passes through the polarizers four times. The polarizer-free type thus can produce brighter display. Since this type can also incorporate a reflector electrode in a liquid crystal cell, this type produces a display free from parallax. In a case of color reflection type LCD employing a color filter, in particular, the probability where an incident light and outgoing light travel different color regions is almost minimized. This also contributes to producing a brighter display.
A display principle of the reflection type LCD is described hereinafter with reference to the accompanying drawings.
FIG. 9 illustrates a display principle of the conventional "polarizer-free type" reflection LCD. When the power is OFF, incident light to the liquid crystal panel is scattered according to a difference between the refraction factors of the liquid crystal and polymer. Further, the incident light is reflected by the mirror-finished surface of reflector electrode, and diffusely reflected as shown in FIG. 9. On the other hand, when the power is ON, light-scattering-liquid-crystal becomes almost transparent, and most of the incident light is reflected like by mirror. At this time, display including intermediate key can be seen by a viewer "A", because brightness is modulated between the respective reflected brightness "A-on" and "A-off" at power ON and OFF. The reflected brightness at power OFF, corresponding to white, less depends on an angle. Further, not only a higher reflected brightness can be obtained because of no absorption by the polarizing plates but also a high purity white, proper to the light scattering liquid crystal, can be obtained. As a result, an excellent visibility similar to that of "paper" can be realized.
However, in this conventional reflection type LCD, an irregular display on the reflected brightness at power ON, corresponding to black, is observed through some direction. A viewer "B" in FIG. 9 sees intense mirror-reflected light "B-on" because the viewer "B" is located in the regular reflective direction and the light scattering liquid crystal becomes transparent when the power is ON corresponding to black. A reflected brightness "B-off" on white level at the viewer "B" has less brightness than the reflected light "B-on", therefore, brightness order in an image is reversed, i.e. reverse image is produced. The area where such an irregular display occurs approximately covers an angle area of 10.degree. in the regular reflecting direction with regard to the incident light.